Tracking of an item of interest in the image is desirable in various imaging applications. For example, in controlling the position of a camera for star tracking, the camera may adjust to center on a star. Similarly, it may be desirable for a camera for video conferencing to track a moving object such as a speaker. In order to track the item, an imaging system typically analyzes a captured frame of image data to determine the location of the center of mass (“COM”) or centroid of the image. In an image, the COM is the location in the image which corresponds to the average light intensity, weighted by location. The camera then typically tracks that COM location.
In imaging systems which use an active pixel sensor (“APS”) for image capture, the APS is typically fabricated with a CMOS process. However, conventional CMOS sensor imaging systems which identify the COM are typically limited in their imaging capability.
In research by Shibata, capacitor scaling was used for identifying the COM in a relatively small number of inputs, such as 25 inputs. However, capacitor scaling for a large number of inputs, such as in a large image, using this technique is not very practical.
In research by Deweerth and Mead, a 1D photoreceptor array which computed the COM in an analog circuit was introduced. Deweerth proposed a 2D extension of this array in which the receptors were alternated spatially so that the currents from adjacent receptors were added to opposing axes. The resolution was limited in the 2D case to be half of that of the 1D case. In addition, the circuitry for calculating the COM resided in each pixel and contributed to both axes. Thus, the fill factor was lower than in the 1D case.
Furthermore, the circuit only tracked the COM, regular readout imaging was not available through the same imager.